In Multiple-Input Multiple-Output (MIMO), multiple transmission antennas and multiple reception antennas are used. By this method, data transmission/reception efficiency can be improved. That is, since a plurality of antennas is used in a transmitter or a receiver of a wireless communication system, capacity can be increased and performance can be improved. Hereinafter, MIMO may also be called “multi-antenna”.
In the MIMO technique, a single antenna path is not used for receiving one message. Instead, in the MIMO technique, data fragments received via several antennas are collected and combined so as to complete data. If the MIMO technique is used, a data transfer rate may be improved within a cell region having a specific size or system coverage may be increased while ensuring a specific data transfer rate. In addition, this technique may be widely used in a mobile communication terminal, a repeater and the like. According to the MIMO technique, it is possible to overcome a limit in transmission amount of conventional mobile communication using a single antenna.
The configuration of the general multi-antenna (MIMO) communication system is shown in FIG. 1. NT transmission antennas are provided in a transmitter and NR reception antennas are provided in a receiver. If the multiple antennas are used in both the transmitter and the receiver, theoretical channel transmission capacity is increased as compared with the case where multiple antennas are used in only one of the transmitter or the receiver. The increase of the channel transmission capacity is proportional to the number of antennas. Accordingly, transfer rate is improved and frequency efficiency is improved. If a maximum transfer rate in the case where one antenna is used is Ro, a transfer rate in the case where multiple antennas are used can be theoretically increased by a value obtained by multiplying Ro by a rate increase ratio Ri. Here, Ri is the smaller of the two values NT and NR.
For example, in a MIMO communication system using four transmission antennas and four reception antennas, a transfer rate which is four times that of a single antenna system can be theoretically acquired. After the theoretical capacity increase of the MIMO system was proved in the mid-90s, researched into various techniques of substantially improving a data transfer rate has been actively conducted up to now. Among them, some techniques have already been applied to various wireless communication standards of third-generation mobile communication and a next-generation wireless Local Area Network (LAN).
The MIMO technique may be divided into a spatial diversity scheme for increasing transmission reliability using the same symbols passing through various channel paths and a spatial multiplexing scheme for transmitting a plurality of different data symbols using a plurality of transmission antennas so as to improve a transfer rate. In addition, these schemes are adequately combined so as to obtain respective merits.
In association with the MIMO technique, various research such as information theory associated with MIMO communication capacity computation in various channel environments and multiple access environments, research on radio channel measurement and model derivation of the MIMO system, and space-time signal processing technology for improving a transfer rate and improving transmission reliability have been actively conducted.